It is generally desirable to keep iron concentrations in BWR feedwater at a low level, typically within the range of 1 ppb±0.5 ppb Fe. In the past, this has been achieved with improvements in condensate clean up systems, and reduction of carbon steel corrosion in the condenser, feedwater piping and heat exchangers. Some plants have been very effective and now have lower than the optimum level of 1 ppb±0.5 ppb Fe. Total iron for a typical plant is approximately 100 Lbm (pounds mass per minute) Fe per year.
In the past, optimum levels of iron have been achieved by injecting iron in the feedwater stream. Direct injection of a solution, with soluble iron or iron particulates, has been proposed. However, these methods generally require an active pumping approach, and have been rejected for use with BWR's due to their complexity and lack of reliability.
Another approach has been to employ a passive pellet dissolution system recirculating hot water around the final feedwater pump. This also has not met with acceptance.
A further proposal has been to incorporate iron in the zinc system as iron pellets or compounds of zinc-iron. The potential problem with this approach is that the zinc system only allows one or the other to be controlled. For acceptable control, plants should be able to exercise independent control of both compounds.
Some plants have used condensate clean-up system bypass flow to elevate the feedwater iron. However, this is not an acceptable approach as it allows undesirable chemicals to bypass the condensate clean-up system.